Photon assisted electron emission from solids have been investigated extensively since early 1960. Specially with the advances of Q-switched and Ti-Sapphire ultrafast laser sources, the vast majority of researches devoted to demonstrating sharp electron beam via exploring the electron emission from various metallic surface or sharp tip emitter exposed to high power picosecond laser radiation. Ultrafast electron source is essential for many applications such as free electron laser source, vacuum electronic high-power THz generation and ultrafast electron microscopy. Up to date, intensity dependence of the photon assisted electron emission as well as energy distribution and emittance of the emitted electrons were at the top interests of the published papers. Despite all developments achieved in more than 50 years after the initial results on photon assisted electron emission, this phenomenon suffers from low quantum efficiency as the biggest challenge for being used in wide range of the abovementioned applications. The recent value of the reported QE for Cu used as photocathode in RF photo-gun is around 1e−5 and it hardly exceeds slightly over 1e−4 only under deep UV (at 250-300 nm) illumination.
Realization of on-chip, low-power, high-speed, spatially addressable electron emission arrays would be potentially transformative for a variety of civilian and military applications, including but not limited to, electron microscopy, electron beam lithography, space propulsion, high power microwave (HPM) devices, free electron lasers, displays, and ultrafast electron diffraction. While electrically-gated field emission devices have been heavily explored in the past, the large capacitances due to the close proximity of a control gate often limits the maximum modulation frequency of these devices. Optical modulation of emission offers the highest modulation speeds as well as a variety of emission mechanisms such as single photon photoemission, multiphoton emission, and thermionic emission. However, in general, optical approaches rely on free-space coupling of an optical beam onto electron emitters, a process that is highly inefficient, particularly when utilizing nanostructured tips. Furthermore, free-space coupling to nanostructures places stringent requirements on incident laser alignment, and is not practical if nanoscale alignment between incident photons and arrays of millions of emission tips is required.
Accordingly, there is a need for improved methods for generating electron emission currents.